The more I look at the Ultimate spirit selection, the weirder it gets for me, cause there are some really weird gaps or overrepresented parts of a series via Spirits. I'll use Kirby, Fire Emblem, and Mega Man as primary examples.
Comparing them to Mario, which has practically every part of its series represented in some capacity, these series have some very noticeable holes. Kirby is the most simple so I'll start with it. Once again, it has a very clear Kirby Super Star bias, as a bunch of characters from those games are Spirits. Like, how is ****ing Plasma Wisp in any way shape or form relevant? But that aside, the modern games get shafted as they have like 1, maybe 2 spirits representing each game. This is compared to the large number from older games. They also included ****ing Gryll which was a bit weird when other more notable characters like Shadow Kirby, DMK, and Drawcia just don't show up at all. Really weird exclusions and inclusions on that end, not as bad as other series at least.
Fire Emblem, despite having a very wide supporting cast, ironically has some of the worst representation of certain games. The Valentia games literally only have Alm and Celica, the Elibe games have a very odd selection, and the Arachnea spirits are very oversaturated compared to everything else (Did we really need Bord and Cord and Barst?). Most games hardly get over five characters from each one and thus miss out on a lot of key characters like Zephiel, Arvis, and Myrrh. Even the games playable characters are from don't get that many all things considered.
Mega Man is super weird. Despite the main focus being the Mega Man X series nowadays, Mega Man X has hardly any Spirits at all, most of them are from Classic Mega Man (Which also randomly excludes a bunch of later installments) Like off the top of my head all they have is X, Zero, and Sigma and no one else. Not to mention like every other Mega Man series has like, maybe 2 spirits max.
Another thing is that the more recent the series is in Smash, the less Spirits they have. Like Spirit totals from series like Punch Out and Metroid is honestly really small. The spirit distribution is really wonky overall.